Work vehicles, such as tractors and other agricultural vehicles, often have hydraulic lines, sometimes termed an electro-hydraulic remote, to supply hydraulic power to ancillary equipment, or more particularly to a hydraulic load. Two hydraulic lines are generally used, one to supply hydraulic fluid under pressure to the hydraulic load and the other acts as a return line for the fluid discharged by the hydraulic load. Each of these two lines is connectible by a coupling to a hose leading to a respective side of the hydraulic load.
The hydraulic load may be, e.g., a hydraulic cylinder. In such a case, the hydraulic load may be required to extend a rod, retract the rod, lock it in a fixed position, or allow it to float freely. To achieve this, a five port, four position spool valve may be used. Such a spool valve includes two output ports, two input ports, and a load sensing port. The output ports are connected to opposing sides of the hydraulic load, and the input ports are connected to a hydraulic pump (supply port) and a tank or reservoir (return port). The load sensing port is connected to the return port when the cylinder is locked or floating. When the jack is being extended or retracted, the load sensing port may be connected to the supply port.
A pump or special valve may be provided to allow a pressure difference to be fixed between the supply port and the load sensing port. Accordingly, a load sensing pressure may be developed in the load sensing port indicative of the resistance offered by the load. If the load is low, the pressure measured at the load sensing port will be lower than the pressure at the supply port. However, when the load offers high resistance, the load sensing port pressure may be nearly equal to the pressure at the supply port.
Within the spool, a throttle may be provided in the connection leading from the return port to the respective output port. The throttle connected to the return port provides a resistance for a return path. There needs to be resistance in the return path to allow for the fact that the load does not always offer a positive resistance and can instead operate in, e.g., a draft mode. Supposing for example that the hydraulic cylinder is being used to raise a heavy weight. The force to extend the rod is resisted by the weight being raised and the rod can only extend relatively slowly. However, when the spool valve is moved to a position to retract the rod and lower the weight, instead of opposing the movement of the hydraulic cylinder, the weight will assist it (i.e., a negative resistance). In the absence of some form of hydraulic damping or resistance, the weight may drop too rapidly. The throttle is therefore included in the spool to provide resistance in the return path in order to damp the movement of the rod when it is operating in a draft mode.
However, when driving a load that is offering a positive resistance, there should ideally be no resistance to a return flow leading to the tank. Any resistance to the flow offered will cause a pressure drop and reduce the efficiency of the hydraulic system. In particular, the throttling effect in a return path in the spool valve will result in undesirable energy losses.
Accordingly, a hydraulic system that may provide an alternative path for hydraulic fluid between the tank and hydraulic load with less resistance to increase the system's efficiency would be beneficial. Moreover, a hydraulic valve for such a hydraulic system that could quickly respond to the need to bypass the resistance offered by the spool valve would be particularly useful.